1. Field of the Invention
The present invention relates to a defect evaluation apparatus that utilizes positrons. In particular, the present invention relates to a defect evaluation apparatus utilizing positrons that is more compact and has an improved accuracy of measurement as compared to prior art apparatuses.
2. Description of the Related Art
In recent years, as a method for externally detecting and evaluating the electronic structure of materials and the concentration and type of lattice defects existing in materials, a method of utilizing the phenomenon of positron annihilation has attracted attention. When a positron is implanted into a sample, the positron annihilates with an electron to emit, generally, two γ rays. By measuring and analyzing the lifetime of the positrons and the energy distribution and angular distribution of γ rays emitted by positron annihilation, lattice defects and the electronic structure of materials can be studied.
The features of methods utilizing the phenomenon of positron annihilation include high sensitivity to lattice defects, in particular vacancy-type lattice defects, the capability of studying the electronic state of defects or bulk, non-destructiveness, and few experimental constraints such as sample temperature, electrical characteristics, and the like. Therefore, analytical methods utilizing positron annihilation are suitable for evaluating bulk materials and have been used in evaluating a variety of materials such as metals, semiconductors, polymers, and the like. On the other hand, if the energy of positrons can be controlled, the depth to which the positrons are implanted can be optionally selected and positron annihilation can be applied to research on near-surface regions of materials. This technique has been rapidly developed since the 1980s and has attracted attention as a method for evaluating near surface regions of materials.
Although various materials are studied by analyzing defects using a variety of electrical measuring methods other than positron annihilation methods, such as electron spin resonance (ESR), light absorption, and the like, the type of defect cannot be directly identified with any of these methods. In contrast, positron annihilation methods can directly indicate whether the type of defect being observed is a vacancy-type defect. This is the distinct feature of this method.
Positron annihilation methods are used in detecting vacancy-type defects regardless of doped elements, conductivity, or the charge state of the defect (however, positively charged defects are not detected), and are effective in detecting mono-vacancies and multi-vacancies. For example, the approximate positron lifetime in each of perfect crystal, mono-vacancies and multi-vacancies of a variety of semiconductors such as Ge, Si, GaAs, InP, InSb, and the like are substantially proportional to volume per atom (inversely proportional to the valence electron density). Of course, although the details of positron lifetime values depend on the charge states of vacancies, lattice relaxation and the like, the fact that the approximate value is given by such a simple parameter is an excellent feature as a means to detect vacancies.
The principle of lattice defect detection using positron annihilation will be explained below.
Positrons are supplied by β+ decay of radioisotopes or the production of positron-electron pairs from high energy photons. As generally used β+ decay-type radioisotopes, there are 22Na (half-life of 2.6 years), 58Co (half-life of 70.8 days), 64Cu (half-life of 12.7 hours), 11C (half-life of 20.4 minutes), 13N (half-life of 10 minutes), and the like. Positrons from β+ decay have a continuous spectrum whose maximum energy is on the order of from 0.5 MeV to 2 MeV. Electrons and positrons can be pair-produced upon transmission of high intensity gamma rays through a heavy metal target, generated by bremsstrahlung of high energy electrons having an energy of an order of 100 MeV accelerated by an electron linac and directed onto the heavy metal target. In this case, the energy of the pair-produced positrons exhibit a continuous energy spectrum broadened to a maximum value on the order of the acceleration energy of the electrons. Generally, a β+ decay radioisotope is used as a positron source. For example, because 22Na has a long half-life and is relatively easy to obtain and handle, it is used in the form of 22NaCl or the like, as the source. 22NaCl is usually enclosed in a capsule formed from an extremely thin titanium foil or the like, and held by a source holder as described later.
When positrons are incident on a material, they rapidly lose their kinetic energy due to ionization, phonon excitation and the like, and achieve thermal equilibrium with the lattice. The process wherein positrons achieves such a thermal equilibrium state is known as thermalization, and positrons are thermalized within the order of 10−12 seconds. The thermalized positron diffuses into the material until it annihilates with an electron. The time period until thermalized positron annihilates with an electron is on the order of 10−10 to 10−7 seconds, at which time two γ rays having energy of about 511 kev are emitted by one pair-annihilation event. In the diffusion process from thermalization to pair-annihilation with an electron, positrons receive repulsive forces from the nuclei that form the material due to Coulomb interaction. As a result, in perfect crystals, positrons exist in interstitial positions, and the amount of positrons which annihilate with free electrons is higher than that of positrons which annihilate with core electrons in the case of metals, while the amount of positrons which annihilate with valence electrons is higher than that of positrons which annihilate with core electrons in the case of semiconductors. Also, if vacancy-type defects exist, the amount of positrons trapped at vacancy-type defects is higher than that of positrons trapped at interstitial positions, and thus positrons are selectively trapped at vacancy-type defects and they annihilate with electrons. Consequently, the time period from when a positron is injected into a material until they annihilate with electrons varies according to the concentration and the form of defects. Accordingly, by determining the time period from when each positron is injected into a material until it annihilates, that is, the positron lifetime, the defects existing in the material can be evaluated.
FIG. 1 shows a process wherein a positron emitted from a radioisotope (22Na) is injected into a material and annihilates with an electron. When 22Na is used as the positron source, 22Na emits γ rays of 1.28 MeV upon β+ decay, therefore these γ rays can be used as a signal to indicate that positrons are injected into a sample. When positrons are generated using 22Na, after firstly detecting γ rays of 1.28 MeV simultaneously generated with positrons, the time when γ rays of about 511 keV emitted by pair-annihilation of each positron incident on the sample with an electron is detected is measured. By determining differences in the detected times of the γ rays of 1.28 MeV and the γ rays of about 511 keV, a positron lifetime spectrum can be obtained.
Also, if electrons are moving prior to annihilation, the energy distribution of the γ rays is broadened by the Doppler effect, because their energy and momentum are conserved before and after pair-annihilation. Therefore, by measuring the energy distribution of the annihilation γ rays, the momentum distribution of the electrons annihilated can be measured. The energy Eγ of the annihilation γ ray is shown in the following Eq.:                               E          ⁢                                           ⁢          γ                =                              E            o                    ⁡                      [                          1              ±                                                v                                      2                    ⁢                    c                                                  ⁢                cos                ⁢                                                                   ⁢                θ                                      ]                                              (        1        )            wherein, ν is the velocity of an electron, c is the velocity of light, θ is the angular deviation between the γ rays and the movement direction of the electron, and Eo=moc2=511 keV (mo is the rest mass of electron). If the component along the emission direction of the γ ray of the momentum of the electron with which positron annihilates is given as PL, Eq. 1 can be transformed as follows.                               E          ⁢                                           ⁢          γ                =                                                            m                o                            ⁢                              c                2                                      ±                                          cp                L                            2                                =                                                    m                o                            ⁢                              c                2                                      ±                          Δ              ⁢                                                           ⁢              E                                                          (        2        )            
Eq. 2 shows that the broadening of energy around 511 keV corresponds to the momentum component of the electron.
Even if the current highest level detector is used, the extent of Doppler broadening is only approximately two to three times the resolution thereof. Therefore, changes in Doppler broadening are generally evaluated using the ratio of the counts in the central region to the total counts of the Doppler broadening, given as the S parameter. The value of the S parameter increases with the sharpening of the Doppler broadening.
FIG. 2 shows conditions where positrons are trapped at vacancy-type defects in metal taken as an example. As shown in FIG. 2(a), because positrons receive repulsive forces from the nuclei due to Coulomb interaction as described above, they exist in interstitial locations. On the other hand, as shown in FIG. 2(b), if vacancy-type defects exist, the probability that positrons will localize at the defects increases. As described above, annihilation γ ray Doppler broadening reflects the momentum distribution of the electrons with which positron annihilate, and within the defects, the Doppler broadening is sharper than that in the bulk, because the annihilation probability of positrons with core electrons having broad momentum distribution is lower than that in the bulk. Thus, the S parameter is large. As shown in FIG. 2(c), this tendency increases with the increase of the void size of the vacancy-type defects. The concentration and type of the vacancy-type defects can be evaluated by determining the magnitude of the S parameter. Further, within the vacancy-type defects, the lifetimes of electrons are longer than that in the bulk because the electron densities within the vacancy-type defects are lower than that of the bulk.
Consequently, the momentum distribution of electrons can mainly be obtained by measuring the Doppler broadening of annihilation γ rays, and then the electronic state in the vicinity of positron annihilation positions can be evaluated, and also the vacancy-type defects mainly in annihilation positions can be evaluated by measuring the positron lifetimes. Further, complex defects of vacancy and impurity can be evaluated by these techniques.
A prior art defect evaluation apparatus using such positron annihilation phenomenon is well known in the art and is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-270598, for example.
The positron generating apparatus disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-270598 comprises a source section for generating positrons, a moderator (also referred to as “moderating material”) for decelerating the positrons generated by the source section, a sample section in which a semiconductor detector for detecting annihilation γ rays is disposed and the positions are directed on a sample, a transfer section for guiding the positron beam decelerated by the moderator into the sample section, a shielding section disposed in the vicinity of the source section, an acceleration section for accelerating the positrons, a beam position fine adjustment section for correcting the positron beam, a discrimination section for eliminating high speed positrons and electrons, and a linearizing section for converting a pulsed beam to a direct current. This positron generating apparatus made it possible to focus a number of positrons by arranging a plurality of radioisotopes and curving the structure of the moderator. Further, this apparatus made it possible to reduce exposure during maintenance by providing shielding plates for shielding positrons and γ rays generated from radioisotopes during maintenance and an infrared radiation generating apparatus for annealing the moderator inside the positron generating apparatus.
In precisely evaluating lattice defects in the vicinity of the surface of materials to be measured using positrons, it is necessary to use slow positrons with monochromatic energy distributions. In order to obtain such slow positrons with monochromatic energy distributions, generally, high energy positrons with large energy distributions (0 to 0.5 MeV) emitted from β+ decay radioisotopes of 22Na or the like as described above are injected into a metal such as tungsten (W) or the like, referred to as “moderator”, and extracted within a vacuum after removing their energy by inelastic scattering within the moderator. The conditions required for a moderator to effectively obtain desired positrons are that the moderator is defect-free, that the surface is clean and that the position work function is negative. Generally, because lattice defects will be introduced into the moderator and the surface cleanness will be reduced during long periods of use, the moderator must be periodically annealed. Further, in a defect evaluation apparatus using positrons, it is necessary for the entire path of the positrons including the radioisotopes, the moderator, and the sample to be measured to exist in the same vacuum from generation of the positrons till their arrival at the sample to be measured.
Conventionally, in order to obtain a defect-free moderator with a clean surface, the moderator is annealed by heating it at a temperature which istypically between 2000 and 2500° C. after removing it from the vacuum formed within the defect evaluation apparatus. However, when this operation is performed, until the moderator is reinserted in the positron generating apparatus after annealing, defects introduced into the moderator by impacts or the like due to attaching the moderator to a pedestal in the air cannot be avoided, and contaminants in the atmosphere attaching themselves to the surface of the moderator cannot be avoided until the moderator is reinserted into the defect evaluation apparatus and restored to a vacuum state. Also, even when the moderator is annealed without being removed from the defect evaluation apparatus, because the annealing temperature is high, the source, other equipments required for measurement, the vacuum container, and the like must be protected from the heat generated during annealing, therefore a mechanism for substantially separating the moderator from the source or a mechanism for thermally protecting the source and the like must be provided, which inevitably makes an increase in the scale of the apparatus.
In particular, when a radioisotope is used as a source and the moderator is annealed without removing it from the defect evaluation apparatus, the source must be protected from the radiation of photons from the heating device, since radioactive leakage must not occur and degeneration of the source due to the heat is not desirable, the source requires protection from heat. Even in a defect evaluation apparatus that utilizes high energy positrons generated upon transmission of high intensity γ rays through a heavy metal target, generated by bremsstrahlung of high energy electrons accelerated by an electron linac and directed onto the heavy metal target, it is necessary to prevent temperature increases in the heavy metal target and other components for supporting the heavy metal target (such as insulation components that are not heat resistant) when the moderator is being annealed.
Further, as a prior art heating apparatus used for annealing a moderator, there is one which uses infrared rays, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-270598, but this is insufficient for uniformly annealing the moderator at a high temperature, typically of from 2000 to 2500° C., necessary for lowering the lattice defect concentration.
Also, in prior art defect evaluation apparatus utilizing positrons, there is generally a disadvantage that the noise level increases and the signal to noise (S/N) ratio decreases with the reduction in the size of the apparatus.